Cleanliness in respect to _bedclothes_ is very important. Organic matters evolved from the skin, lungs, etc., hang about the bed-linen, and give the bedroom the “close smell” which can be perceived on entering it in the morning straight from the fresh air. The beds should not be made directly after being evacuated, but the clothes should be thrown over the bottom of the bed, the bolsters and mattress well shaken, and every part exposed to a free current of air during the greater part of the morning, before re-arranging the clothes. Eider-down quilts, unless frequently ventilated by exposure to outside air, are unwholesome. Superfluous bedroom furniture should be avoided, as it all takes away from the breathing-space. Bed-hangings should be reduced to a minimum, and all excretory matters covered up during their stay in the room, and removed as early as possible. Cleanliness of _the house_ is also very important as a means of health. Dust, in however obscure a corner it rests, attracts to itself organic matters, and forms a soil in which disease germs may grow. Besides this, it devitalises the air, depriving it of its active oxygen. (See also page 101). Dust in _the streets_ serves to carry about various diseases, besides mechanically irritating any part it comes in contact with, producing bronchitis, etc. CHAPTER XL. CLOTHING. =Physiological Considerations.=—The average temperature of the surface of the body in man is 98·4 to 98·6°. The maintenance of a tolerably uniform temperature is an essential condition of life. The factors governing the temperature of the body are _the amount of heat produced_ and _the amount lost_. If more heat escapes, more has to be generated; and the source of all the heat produced in the body is the food taken. This becomes changed by the metabolic processes occurring in the body which produce heat. =Heat is lost=, (1) by the _skin_; (2) in _respiration_, the expired air having been heated during its stay in the lungs; (3) with the _food and drink_ taken, if not at the temperature of the body; (4) with the _excreta_; and (5) by transformation of heat into _mechanical energy_. Of the whole loss by these different channels, probably eighty to ninety per cent. is through the skin.